This invention relates to a locking and drive unit for a rotating body, in particular for motor vehicle wheels in a balancing machine.
Typically the invention is provided for balancing motor vehicle wheels, however it can also be applied to other rotating bodies, such as flywheels.
The unit to which the invention relates comprises:
for supporting and driving the rotating body, a motorized main shaft carrying flange means which define a counteracting surface for the rotating body;
pressing means coaxial with the shaft to axially press the rotating body against said counteracting surface and to lock it relative thereto;
a traction rod associated with the main shaft and movable axially relative thereto, and connected to the pressing means to pull them against the counteracting surface;
elastic thrust means (consisting for example of a robust precompressed spring) connected to the traction rod to axially pull, via this latter, the pressing means against the counteracting surface to lock the rotating body relative thereto.
A unit having these characteristics for a balancing machine is described in German patent document DE 40 00 424.
In use, when a wheel has to be fitted to the main shaft, the traction rod is firstly moved axially to withdraw the pressing means from the counteracting surface (to the right in the accompanying FIG. 1) by the action of a fixed pneumatic cylinder-piston unit acting on a first end of the traction rod such as to move the rod in the opposite direction to the force generated by the elastic thrust means. The wheel is then positioned on the shaft with the web of its wheel rim placed against the counteracting surface, and suitable pressing means (usually consisting of a conical centering cone and a locking ring nut) are mounted on the shaft in contact with the web, so that this is sandwiched between the counteracting surface and the pressing means, but without substantial axial pressure; the pneumatic cylinder-piston unit is then unloaded to nullify its thrust on the traction rod, so that this, by the action of the elastic means, undergoes traction with a strong thrust such that the wheel web is strongly clamped between the pressing means and the counteracting surface, to an extent such as to achieve reliable rigidity between the wheel and shaft.
However this operation involves risks due to the fact that sometimes, during that stage in which the wheel is manually brought into contact with the contact surface and the pressing means are placed against the wheel, this latter can lock, due to friction, against any obstacle before adhering to the counteracting surface, for example against the cone or against the shaft, and hence not be perfectly in contact with the pressing means; in such cases, when the pneumatic cylinder-piston unit acting on the traction rod is discharged and the rod remains subjected only to the action of the elastic means, the obstacle which had locked the wheel is overcome and energy stored in the elastic means is suddenly released, to be dissipated as a violent impact by the moving parts against the fixed part.
Again, it can happen that the ring nut (which in most cases is of the rapid release type) is erroneously released before the cylinder-piston unit acts on the traction rod to discharge it. Again in this case the energy stored in the elastic means is suddenly released, to be dissipated as a violent impact by the moving parts against the fixed part.
This can result in damage to parts of the machine, in particular to the measurement components and the bearings, or in noise which is absolutely unacceptable in a precision machine.
An object of this invention is to obviate said drawback by a technical solution which is constructionally simple and operationally effective.
This and other objects are attained by the invention as characterised in the claims.
The invention is based on the fact of comprising:
an impact damping means, acting by mutual movement of two of its elements in the same direction as the axis of the main shaft when the traction rod is moved in the sense of pulling the pressing means towards the counteracting surface, a first of said elements being rigid with the main shaft and the other element being rigid with the traction rod.
Said damping means comprises a closed cylindrical chamber, filled with damping fluid and within which a piston is sealedly slidable, said piston possessing a constricted aperture for passage of damping fluid from one side of the chamber to the other when the traction rod is moved in the sense of pulling the pressing means against the counteracting surface, said piston and said chamber being rigid with the traction rod and with the cylindrical chamber respectively, or vice versa.